This invention relates generally to the construction of circuit boards on which large numbers of integrated circuits are mounted. As integrated circuits have become more complex, they have also grown in their need to dissipate power, in the form of heat. A parallel development in integrated circuits has been an increase in their speed of operation, as governed by clock signals distributed to the circuits. For faster clock speeds, the distances over which signals are transmitted become of more critical importance. In general, as the clock speed increases so does the need for spacing various circuits on the same circuit board more closely together. Thus two factors in the design of integrated circuits have combined to increase the power density on circuit boards. Higher circuit complexity results in higher power dissipation levels for each integrated circuit module; and the use of higher clock speeds requires that circuit modules must be more closely spaced on each circuit board.
Until recently, the usual technique for cooling electronic circuitry mounted on circuit boards was to merely blow air through the spaces between parallel, rack-mounted boards, from one end to the other. The difficulty with this approach is that there is inevitably a temperature gradient from one end of each circuit board to the other. For efficient cooling of the more powerful circuit modules, they must be positioned close to the cooler air at the inlet end of a rack of boards. Another problem caused by the presence of a temperature gradient is that, for certain types of circuit chips, such as emitter-coupled logic (ECL) circuitry, a temperature difference between chips degrades their ability to communicate.
One method of increasing the capacity for air cooling of circuit boards is known as impingement cooling. Basically, in an impingement cooling system the cooling medium, usually air, enters through a plenum that is in close proximity to one or more circuit boards. This inlet plenum is positioned between adjacent circuit boards and has openings in its walls to direct air onto selected circuit modules and components. The plenum openings can be of various sizes to ensure that just enough cool air impinges directly on each circuit to be cooled. After impingement on the circuit modules, the cooling air flows in a direction parallel to the boards, as in a conventional cooling arrangement.
There are a number of patents disclosing impingement cooling systems. For example, Bell, U.S. Pat. No. 4,498,118 discloses a structure in which a cooling plenum is mounted between two adjacent circuit boards, and has openings to permit cooling air to impinge directly on components mounted on the circuit boards. Similarly, U.S. Pat. Nos. 4,277,816 to Dunn et al. and 4,296,455 to Leaycraft et al. disclose a cooling system in which a separate air plenum has openings positioned to provide cooling flow to selected components on an adjacent circuit board, and each opening has a cylindrical sleeve mounted in it, to carry the air directly onto heat sinks of the components, which are appropriately spaced to permit spent air to have minimal effect on adjacent circuit modules.
Prior impingement cooling systems have used relatively large air plena located between adjacent circuit boards. This increases the overall volume required to house the circuit boards, and lengthens signal paths needed for board interconnections. Attempts to use the same cooling principles with thin air plena have resulted in high pressure losses, and require noisy, oversized blowers.
Another difficulty with larger circuit boards is that they tend to lack the degree of stiffness needed to facilitate handling during installation and removal. Various structural modifications have been proposed to increase board stiffness, the most common being the addition of stiffening bars fixed to each board. The major difficulty with these is that they interfere with the flow of cooling air, and they restrict component positioning.
Accordingly, there is still need for improvement in the construction of impingement air cooling systems for circuit boards. The present invention is directed to this end.